Conventionally, transfer molding is employed for encapsulating electronic devices wherein molding compound is introduced as a solid pellet into a mold supply pot of a molding system and melted with the application of heat and pressure to a liquid state. The liquefied molding compound is then forced by a plunger into runners connected between the plunger and molding cavities to enter into molding cavities via narrow gates.
In modern packaging technology, semiconductor chips or dice are becoming increasingly thinner. The separation distances between wires connected to the semiconductor dice are also becoming smaller. Moreover, more complicated structures such as stacked dice have been developed. In these cases, if the height of the molding cavity is low and close to a die surface, such that transfer pressure is increased, a high molding compound injection speed resulting from conventional transfer molding will damage the dice or wires. The narrow gap increases resistance to the flow of the molding compound such that voids may arise in the resulting molded package. The problems encountered in transfer molding are especially evident when the gap between the top surface of a die and a top molding surface is reduced to less than 0.1 mm. Furthermore, a wider molding cavity to mold larger dice would make it more difficult for molding compound to perfectly fill the molding cavity. In such cases, the further the molding compound is propelled away from the gate, the more difficult it is for the ejected liquefied molding compound to properly fill the molding cavity.
To overcome the aforesaid problems associated with transfer molding of thin and/or wide packages, one approach is to conduct molding by dispensing liquid molding compound directly on top of the electronic devices to be molded, and then compressing the molding compound to encapsulate the electronic devices and to form the desired shape of the electronic package in a process known as compression molding. U.S. Pat. No. 6,743,389 entitled “Resin Molding Machine and Method of Resin Molding” describes a compression molding process wherein a spring-biased damper downwardly projected from a resin molding face allows the molding machine to apply a compacting force in a molding cavity after the mold is closed onto the electronic device.
Using compression molding, since molding compound in the form of liquid or paste resin is supplied individually onto each device which is encapsulated, complete filling up of the narrow gap between the dice and the molding cavity can be achieved without the above problems associated with transfer molding. Nevertheless, transfer molding remains a very popular molding method, since it is a cheap and efficient way to distribute molding compound from a single solid pellet to multiple molding cavities. In compression molding, liquid or paste resin has to be individually delivered to the multiple cavities. It would therefore be desirable to provide an improved transfer molding technique to achieve the respective benefits of transfer molding and compression molding in the same system.